Is Facebook preparing to hurdle the Great Firewall?

"It was just two nerds comparing notes," the spokesman said. "Keep the speculation in check."

But when those nerds happened to be the Facebook founder Mark Zuckerberg and Robin Li, the head of Baidu – the biggest search engine in China – there was no way a quiet business lunch was going to remain quiet.

Moments after Zuckerberg and Li were seen strolling through the canteen in Baidu's Beijing headquarters today, an employee posted a blurred mobile phone photograph of them on his microblog.

The image spread quickly, first via Chinese social networking sites, then on to the English language side of the internet, prompting speculation that the two IT players may be planning to cross the divide.

Zuckerberg – recently named person of the year by Time Magazine – has made no secret of his desire to expand in China, where Facebook has been blocked by the government censors' Great Firewall since 2008.

On a recent global map of Facebook users, China appeared as a black spot, though it has a bigger internet population than any country on Earth.

Zuckerberg's current holiday is his first known trip behind the Great Firewall. But he has started taking Mandarin lessons, and recently asked Facebook members for tips on places to visit with his girlfriend, Priscilla Chan.

In a recent speech at Stanford University, he said the company may turn its attention to China in a year if it can first crack Japan, South Korea and Russia.

"How can you connect the whole world if you leave out a billion people?" he asked then.

"Our theory is that if we can show that we as a western company can succeed in a place where no other country has, then we can start to figure out the right partnerships we would need to succeed in China on our terms," he said.

Zuckerberg appears to have found common ground with Li, an internet entrepreneur who has completed a postgraduate course in the US.

Since then, he has shrugged off Google and Yahoo, as well as criticism about a supposedly weak stance on censorship and copyright piracy, to make Baidu the dominant force in the Chinese search engine market.

In an earlier interview with the Guardian, Li said Baidu would one day become an international rival to Microsoft and Google.

Since the two men were introduced in autumn 2009, they have met twice before today, and are said to have hit it off.

Kaiser Kuo, Baidu's director of international communications, said he was not privy to the details of their latest discussion.

"As far as I know, this was two nerds comparing notes," he said.

Any talks are likely to be exploratory. Given the furore over censorship that followed Google's decision to curtail its Chinese search engine earlier this year, it is unlikely Facebook and Baidu would like to draw further attention to the issue. China already has two social networks that are Facebook imitators: Kaixin, with 80 million users, and Renren, with 150 million.

These lack the economic clout and global reach of Zuckerberg's company but they do have the advantages of language and cultural awareness, as well as the protection of the Great Firewall.

To tackle them and other big Chinese platforms, such as QQ, Facebook would probably have to move inside the firewall and accept greater censorship.

"If Facebook wanted to enter China, it would not have to change its function, because netizens here are used to copycats already, but it must, like other international internet companies, obey Chinese laws and regulations," said Hu Yong, a professor at Beijing University's school of journalism and communication.

Critics already accuse Facebook of failing to protect its members' privacy settings and selling personal data to advertisers. If it was to also sacrifice free debate in order to secure a share of the Chinese market, it is likely to come under even greater fire for lacking business ethics.

The sensitivities are acute. Earlier this year, a group of Chinese activists wrote an open letter to Zuckerberg after their Tiananmen Square protest memorial page on Facebook – "Never Forget June Fourth" – was closed down. They expressed concern that they were being harassed to placate China's censors, but Facebook administrators insisted the action had been taken because they set up the wrong kind of page.

Currently, there are numerous sites on Facebook that would not be permitted in China, including clubs and groups that campaign on behalf of the victims of Tiananmen, the imprisoned Nobel laureate Liu Xiaobo and Falun Gong.

The Dalai Lama, who is reviled by the Chinese authorities, has an official Facebook page that is "liked" by more than a million people. This would be impossible if the company was to move its servers inside China.